Iridium (III) complexes (Non-Patent Documents 1-4) are known to be phosphorescent and are expected to be applied to the organic EL display, etc.
On the other hand, the present inventors have discovered an iridium complex ((btp)2Ir(acac)) that emits oxygen concentration-dependent phosphorescence, and have developed the method for measuring oxygen concentration in biological tissues using the room-temperature phosphorescence (intensity and lifetime) of the iridium complex ((btp)2Ir(acac)) (Patent Document 1). They also succeeded in determination of oxygen concentration within liposome membrane, the visualization of oxygen concentration within cancer cells, and the tumor visualization of a tumor-bearing mouse by measuring the phosphorescence intensity and lifetime of (btp)2Ir(acac) (Patent Document 1).
In addition, a compound that emits phosphorescence in the near-infrared area in a oxygen concentration-dependent manner has been developed (Patent Document 2). A water soluble phosphorescent compound has also been developed (Patent Document 3).
The luminescent probe method using the iridium complex as described above is very useful to the noninvasive and super-sensitive measurement of the oxygen concentration in a specific part of a microstructure like cells. In general, the method for measuring the oxygen concentration with a luminescent probe utilizes the probe molecules whose luminescence is quenched by collisions with oxygen molecules; that is, the luminescent probe whose luminous intensity changes depending on the oxygen concentration. However, the method for determining the oxygen concentration based on the change of the luminescence intensity gave an accurate value when the concentration of the probe molecule and the excitation-light intensity distribution are uniform, while there is a problem that analysis becomes difficult because of the influence of the concentration when the concentration distribution of the probe molecule is not uniform such as the measurement of the oxygen concentration in a cell.
Therefore, a method based on the change of a luminescence lifetime is considered as a method that is not influenced by concentration, but there is such a drawback that a device becomes large-scale because an expensive light source such as a pulse laser and advanced light measurement technique are generally required for the measurement of the luminescence lifetime.